The present invention relates to a stop maintaining system of a vehicle, particularly to a stop maintaining system of a vehicle which is capable of reducing generation of creep groan.
As conventional brake devices of vehicles, foot brake devices which brake vehicle wheels by applying hydraulic brake pressure to hydraulic brake mechanisms according to a depression of a brake pedal, brake force control devices which include a pressurizer for increasing hydraulic brake pressure applied to hydraulic brake mechanisms, and brake vehicle wheels by controlling the pressurizer independently from the depression of a brake pedal, and electric parking brake devices which brake vehicle wheels by operating electric brake mechanisms which are driven by electric actuators independently from the depression of a brake pedal, are known.
Further, arts for reducing, when a vehicle is detected to be in a stopped state, a burden on a vehicle driver which accompanies the depression of a brake pedal by operating an electric parking brake device which is represented by an Anti-lock Brake System (ABS) or a Dynamic Stability Control (DSC) system, are proposed.
JP4961857B discloses an automatic brake force control device of a vehicle. While an auto hold control for maintaining a stopped state of a vehicle by maintaining hydraulic brake pressure due to the depression of a brake pedal by a vehicle driver is performed, if an auto hold control release condition (e.g., an accelerator pedal is depressed) is satisfied, the automatic brake force control device sharply reduces the hydraulic brake pressure at a first gradient and then at a timing when a brake force corresponding to the hydraulic brake pressure matches a drive force of an engine, gently reduces the hydraulic brake pressure at a second gradient which is gentler than the first gradient.
Thus, a shock which occurs immediately after starting the vehicle is reduced while lowering a burden on the driver.
Meanwhile, in order to improve fuel consumption and emission performance, vehicles including an engine automatic stop-restart mechanism are known.
In such a vehicle including the engine automatic stop-restart mechanism, when a stopped state of the vehicle is detected due to a traffic light etc. during traveling, the engine is stopped by cutting fuel supply to the engine, and then when depression of an accelerator pedal is detected, the fuel supply to the engine is started to restart the engine.
In vehicles mounted with an automatic transmission including a torque converter, when shifting from a stopped state where a brake pedal is depressed to starting by loosening the depression of the brake pedal, a stick-slip phenomenon occurs between friction surfaces of a brake pad and a rotor disk within a given hydraulic brake pressure range, thus, an abnormal sound (creep groan) occurs.
The creep groan is considered to occur in the following manner. In a vehicle start, energy is accumulated once the hydraulic brake pressure is maintained until around when the brake force corresponding to the hydraulic brake pressure matches the drive force of the engine, while causing an elastic deformation in a friction member of the brake pad.
This accumulated energy is released at once after the brake force corresponding to the hydraulic brake pressure matches the drive force of the engine, and converted into a periodic force variation, i.e., repetition of a static-frictional engagement and a dynamic-frictional engagement. This vibration force causes resonance around the brake and suspension.
The automatic brake force control device of JP4961857B controls a latter part of a release characteristic of the hydraulic brake pressure to be the second gradient which is gentler than the first gradient which is the release rate for an early part of the release. Thus, a jump start of the vehicle is prevented and a starting performance is improved.
However, with the art of JP4961857B, since the second gradient which is the release rate for the latter part is gentler than the first gradient which is the release rate for the early part, a period of time for which the hydraulic brake pressure remains within a stick-slip occurable range within which the repetition of the static-frictional engagement and the dynamic-frictional engagement may occur becomes long and the creep groan may occur for a while.
Although it may be considered to increase the release rate of the hydraulic brake pressure in order to shorten the time for the hydraulic brake pressure to pass the stick-slip occurable range, there is a concern of degradation of the starting performance of the vehicle. Especially with the vehicle equipped with the engine automatic stop-restart mechanism, since a hydraulic brake pressure release condition is the same as an engine restart condition and the hydraulic brake pressure is released at the same time as when the engine restarts, if the release rate of the hydraulic brake pressure is simply increased, the vehicle wheels begin to be driven (the vehicle starts traveling) even though the drive force of the engine is not stabilized, and the engine may stall (cause a misfire).